Martensitic stainless steel is used in applications requiring relatively low corrosion resistance, such as knives, forks, and other table utensils, and generally contains as its components 11.5-14.0% of Cr, a maximum of 0.40% or less of C, 1.0% or less of Si, and 1.0% or less of Mn. A method commonly used to make it involves hot rolling a continuously cast slab or a slab obtained by ingot making and blooming, softening the hot-rolled sheet steel by batch-type annealing, then pickling, cold rolling, and finish annealing to give the product.
The batch-type annealing step used in the above-mentioned conventional manufacturing process to soften hot rolled steel generally takes a long time, typically tens of hours. When a hot-rolled sheet of chromium stainless steel is subjected to such prolonged annealing, a Cr-depleted layer forms on the surface of the hot-rolled steel sheet, presenting a serious problem especially in martensitic stainless steels having a relatively low Cr content. More specifically, formation of a Cr-depleted layer at the surface by annealing of hot-rolled steel sheet deteriorates the oxidation resistance of the sheet surface so that the finish annealing step of the cold-rolled strip production process that follows annealing of hot-rolled steel sheet might cause a thick undesirable scale to form on the surface of the steel sheet, leaving a problem of descaling. Also, after finish annealing, martensitic stainless steel is ordinarily polished as by buffing to give an aesthetic surface before use. But if the undesirable scale resulting from finish annealing remains, buffing work becomes extremely difficult.
To date, various counter measures have been employed for overcoming the problem of Cr-depleted layer formation. One approach is to extend the time of pickling following annealing of hot-rolled steel sheet to fully dissolve the outer surface to remove the Cr-depleted layer, thereby preventing deterioration of oxidation resistance during finish annealing. However, such an approach brings about new problems such as a longer pickling time, higher costs due to an increase in the quantity of pickling solution used, and difficult disposal of wasted pickling solution having a large amount of metals dissolved therein.
Making studies and experiments on ways to prevent formation of a Cr-depleted layer by shortening the time of hot-rolled steel sheet annealing and lowering the annealing temperature, the inventors found that, while merely shortening the hot-rolled steel sheet annealing time or lowering the annealing temperature has an observable effect of reducing the Cr-depleted layer on conventional martensitic stainless steel, the annealing of hot-rolled steel sheet achieves insufficient softening, resulting in cold rolled products which are markedly inferior in mechanical properties, especially workability.
In this regard, it was reported in Japanese Patent Publication No. 57-55787 that even when annealing of hot-rolled steel sheet is carried out for only a short time, ductility and workability are greatly improved by the addition of boron to chromium stainless steel. There remains, however, the problem of a pronounced decline in the corrosion resistance of boron-added steel due to the segregation of boron at grain boundaries.
An object of the present invention is, therefore, to provide a martensitic stainless steel capable of resolving the problems due to Cr-depleted layer formation, in that a hot-rolled sheet steel can be adequately softened even when annealing of hot-rolled sheet steel is substantially shortened to prevent formation of a Cr-depleted layer during the hot-rolled sheet steel annealing, typically the problem of oxidation resistance of cold-rolled steel sheet as well as the problem arising in conventional steels when the hot-rolled steel sheet annealing is of a short duration, namely, the deterioration of the mechanical properties of cold-rolled sheet steel, particularly the deterioration of workability. Another object of the present invention is to provide a practical method for the production of martensitic stainless steel sheet having improved oxidation resistance, workability, and corrosion resistance by short-duration annealing of a hot-rolled steel sheet.